Heretofore, as a light emitting type electronic display device, cited has been an electroluminescent display (ELD). Constituting elements of ELD include an inorganic electroluminescent element and an organic electroluminescent element (hereinafter referred to also as an organic EL element). An inorganic electroluminescent element has been used as a plane type light source, but high voltage with alternating current is required to drive the element.
On the other hand, the organic electroluminescent element is constituted in such a manner that an emission layer containing light emitting compounds is sandwiched between a cathode and an anode. In the above element, electrons and holes are injected into the emission layer and are subjected to recombination, whereby exciton is generated. During deactivation of the resulting exciton, light (fluorescence or phosphorescence) is emitted. Light emission can be realized via application of a voltage of approximately from several V to several tens V. Further, the organic electroluminescent element exhibits a wide viewing angle due to the self-luminescence type, and high visibility, whereby it has attracted attention in view of space saving and portability, because of a completely solid element of a thin layer type.
In the development in prospective application of organic EL elements, an organic EL element capable of efficiently producing luminescence in high luminance at low power consumption is demanded. For example, in Japanese Patent No. 3093796, disclosed is a technique to improve emission luminance and the prolonging lifetime by doping a stilbene derivative, a distyrylarylene derivative or a trisstyrylarylene derivative with a slight amount of phosphor; in Japanese Patent O.P.I. publication No. 63-264692, disclosed is an element possessing an organic emission layer in which a 8-hydroxyquinoline aluminum complex as a host compound is doped with a slight amount of phosphor; and in Japanese Patent O.P.I. publication No. 3-255190, known is an element possessing an organic emission layer in which a 8-hydroxyquinoline aluminum complex as a host compound is doped with a quinacridone type dye.
In the technologies disclosed in the above-described patent documents, it is said that the limit of externally output quantum efficiency (ηext) is 5% since the formation probability of the light emission exciton is 25% because of a formation ratio of singlet exciton to triplet exciton being 1:3 when luminescence from the singlet state exciton is used, and light-outputting efficiency is approximately 20%.
However, investigation on materials capable of emitting phosphorescence at room temperature has been accelerated by the specification described in M. A. Baldo et al., nature, vol. 403, No. 17, pages 750-753 (2000) and U.S. Pat. No. 6,097,147, since an organic El device utilizing phosphorescence emitted from the triplet state of the exciton was reported by Princeton University via the description of M. A. Baldo et al., nature, vol. 395, No. 17, pages 151-154 (1998).
Further, since in the case of an organic EL element utilizing phosphorescence, which has recently been found out, 4 times the emission efficiency are achievable in principle in comparison to an element utilizing the previous fluorescence, the research and development concerning the layer structure and electrode of a light emitting element, together with material development thereof has been done all over the world. For example, many compounds, principally heavy metal complexes such as iridium complexes, are synthesized and investigated; cf. S. Lamansky et al., J. Am. Chem. Soc., vol. 123, page 4304 (2001).
Further, an organic EL element is a completely solid element possessing an organic material film having a thickness of approximately modest 0.1 μm between electrodes, and relates to a technique expected for the next generation flat display and lighting since luminescence can also be achieved via application of a relatively low voltage of 2-20V.
However, since in an organic EL element, the emission mechanism is based on a luminous phenomenon generated by utilizing deactivation from an excited state of an organic material to a ground state, a larger band gap is desired to produce emission in the short wavelength range of blue, blue-green and so forth. Accordingly, high voltage is desired to excite the large gap.
Further, large damage is produced during returning to a ground state since the excited state itself is situated at a high level, and lifetime tends to be shortened in comparison to luminescence of green or red. The pronounced tendency is specifically seen in the case of phosphorescence generated by utilizing luminescence from a triplet excited state.
In order to solve the above-described problems, there appears various techniques, but there is, for example, a technique of producing a high-molecular weight after forming layers of structure fitted in an electroluminescent element, in which a two-functional triphenyl amine derivative having two vinyl groups in the molecule is described, and a polymer cross-linked three-dimensionally via exposure to UV light after forming the compound is formed (refer to Patent Document 1, for example). There is a technique of adding a material having at least two vinyl groups into a plurality of layers, including a method in which a polymerization reaction is conducted via exposure to UV light or heat during film formation of an organic layer before laminating a cathode (refer to Patent Document 2, for example). There is provided a manufacturing method by which AIBN (azoisobutyronitrile) as a radical generator is added into a comonomer admixture having a vinyl group similarly to a material having a vinyl group at the terminal of a phosphorescent dopant to conduct polymerization reaction during film formation (refer to Patent Document 3, for example). And, there is provided another manufacturing method via cross-linkage generated by producing Diels-Alder reaction between two molecules in the identical layer (refer to Patent Document 4, for example).
Any of the above-described techniques is a method by which the polymerization reaction is completed during or immediately after film formation (before providing a cathode), but it is still insufficient in view of durability of an organic EL element in practicality, and further technological studies to improve durability of the element is demanded.
Patent Document 1: Japanese Patent O.P.I. publication No. 5-271166
Patent Document 2: Japanese Patent O.P.I. publication No. 2001-297882
Patent Document 3: Japanese Patent O.P.I. publication No. 2003-73666
Patent Document 4: Japanese Patent O.P.I. publication No. 2003-86371